Planar velocity analysis of diesel spray shadow images

TL;DR

This paper presents a method using ultrafast laser pulses and specialized imaging optics to obtain spatially resolved velocity measurements of diesel spray structures from shadow images.

Contribution

It introduces a novel optical setup combining collimated light, imaging optics, and ultrafast lasers to accurately measure spray edge velocities in diesel injections.

Findings

Successful velocity measurements of spray edge features.

Enhanced spatial resolution in shadowgraphy images.

Effective suppression of out-of-focus light contributions.

Abstract

The focus of this work is to demonstrate how spatially resolved image information from diesel fuel injection events can be obtained using a forward-scatter imaging geometry, and used to calculate the velocities of liquid structures on the periphery of the spray. In order to obtain accurate velocities directly from individual diesel spray structures, those features need to be spatially resolved in the measurement. The distributed structures measured in a direct shadowgraphy arrangement cannot be reliably analyzed for this kind of velocity information. However, by utilizing an intense collimated light source and adding imaging optics which modify the signal collection, spatially resolved optical information can be retrieved from spray edge regions within a chosen object plane. This work discusses a set of measurements where a diesel spray is illuminated in rapid succession by two ultrafast laser pulses generated by a mode-locked Ti-Sapphire oscillator seeding a matched pair of regenerative amplifiers. Light from the object plane, located at the center of the spray, is directed by a set of imaging optics to the detection plane, where it forms a shadow image on the face of a frame-transfer CCD. In this configuration the depth-of-field of the imaging optics limit detected light, effectively increasing the efficiency of signal collection within the object plane and suppressing out-of-focus contributions. Time-correlated images are collected in this manner and analyzed to obtain object plane velocity data for in-focus edge features of the spray.